Reflex sights are optical or computing sights that reflect a reticle image (or images) onto a combining glass for superimposition on the target. The M68 sight is a reflex sight. It uses a red aiming reference (collimated dot) and is designed for the “two eyes open” method of sighting. The dot follows the horizontal and vertical movement of the gunner's eye while remaining fixed on the target. The sight is parallax free beyond 50 meters and thus the shooter can place the dot of a properly zeroed weapon on a target regardless of its positioning the sight tube and hit the target at distances of 50 meters and greater. However, when zeroing the weapon or engaging targets at distances of 50 meters or closer the dot must be precisely centered to ensure accurate zeroing of weapon or accurate fire on targets. Failure to precisely center the red dot in the tube while zeroing the weapon will either cause difficulty in achieving a zero or if the red dot is maintained in the same non-centered position the soldier will have a false zero on his or her weapon and will be unsuccessful when engaging targets be they on a range or on the battlefield.
Parallax is an apparent displacement or difference in the apparent position of an object viewed along two different lines of sight, and is measured by the angle or semi-angle of inclination between those two lines. In the M68 series scopes this is caused by the fact that there are multiple lenses in the scope. Because of this the soldier may be required to make a visual estimation of center when zeroing this scope. This estimation may be difficult to accurately repeat and may be the most common and serious problem encountered by soldiers when zeroing.